Leafy Lettuce Greenhouse Guide: Crisp Heads from Seed to Salad Bowl

Why Lettuce Loves a Greenhouse

Lettuce is the gold-standard crop for beginner greenhouse growers. It germinates in three to five days, reaches baby-leaf size in three weeks, and produces full heads in as little as forty-five days. A single four-by-eight-foot bench can deliver forty heads every month—enough for a family of four plus extras for neighbors. Because lettuce prefers cool temperatures and hates summer heat, a ventilated greenhouse extends the harvest calendar from February to December in most zones.

Greenhouse Style & Orientation

You do not need a high-tech glass house. A simple 6×8 ft polycarbonate kit placed on a level, south-facing spot works perfectly. Position the doorway to the east so morning sun warms leaves quickly, knocking off dew and reducing mildew risk. Install a single roof vent and a side louvre; lettuce likes daytime temps of 60–70 °F and nights around 50 °F. If summer highs exceed 80 °F inside, clip 30 % shade cloth to the ridge—growth slows, but leaves stay sweet.

Best Lettuce Varieties for Protected Culture

Butterhead

• ‘Buttercrunch’—slow to bolt, velvet leaves
• ‘Sunglo’—compact, ideal for troughs

Romaine

• ‘Rouxai’—deep-red outside, green heart, holds two weeks post-harvest
• ‘Parris Island Cos’—classic crisp ribs, heat tolerant

Loose-leaf

• ‘Oakleaf’—cut-and-come-again for thirty days
• ‘Salanova’ multi-leaf types—one cut produces dozens of baby leaves

Order pelleted seed if you use a mechanical seeder; the clay coating dissolves within minutes of watering and prevents doubles.

Soil or Soil-less: Pick Your Medium

Either route works. For classic benches, fill 4-inch-deep trays with a mix of two parts screened compost, one part coconut coir, and one part perlite. Add two tablespoons of organic 4-4-4 fertilizer per gallon of mix. If you prefer hydroponics, drop seedlings into 2-inch net cups filled with hydroton clay pebbles and set them into a recirculating NFT channel. Keep the nutrient solution at 800 ppm and pH 5.8–6.2. Beginners often find soil trays more forgiving.

Seeding Step-by-Step

1. Moisten the mix until it feels like a wrung-out sponge.
2. Sow pelleted seed 1/4 inch deep, two seeds per cell for butterheads, four per cell for baby-leaf mixes.
3. Cover trays with a humidity dome; open vents after germination to prevent damping-off.
4. Place trays on a 70 °F heat mat for forty-eight hours, then move to a bright bench. Remove the dome completely once cotyledons flatten.

Lighting Schedule

Winter daylight drops below ten hours in northern zones, so supplement with a 40-watt LED bar hung 18 inches above the canopy. Run lights fourteen hours daily; lettuce is a long-day plant for vegetative growth, not flowering. Use full-spectrum white LEDs—no fancy bloom switch needed. Electricity cost: roughly one dollar per month per tray.

Watering Science, Not Guesswork

Lettuce roots need oxygen. Water when the top half-inch of mix is dry; in hydroponics, cycle pumps four times daily for fifteen minutes. Always water at sunrise so foliage dries quickly. Use rainwater or de-chlorinated tap; chlorine at 2 ppm can stall growth. If EC of source water exceeds 0.7, dilute with distilled to prevent tip-burn.

Feeding for Sweet, Fast Growth

Lettuce is a leafy crop, so emphasize nitrogen. For soil trays, drench with fish hydrolysate (2-4-0) at 1 Tbsp per gallon once a week after the first true leaves appear. In hydroponics, switch to a calcium-boosted formula after week two; lack of calcium causes the dreaded brown leaf edge known as tip-burn. Add 1 gram of calcium nitrate per gallon if symptoms appear.

Temperature & Humidity Cheat-Sheet

Dropping night temps the final week boosts sugar content and shelf life.

Pest Watch: The Big Three

Aphids

Look for rolling or yellowing leaves. Release 500 ladybugs per 100 sq ft or spray 1 % neem oil at lights-off to prevent leaf burn. Repeat twice, five days apart.

Thrips

Silvery streaks on leaves. Hang blue sticky cards just above canopy; replace weekly. Introduce predatory mites (Amblyseius cucumeris) if counts exceed five per card.

Downy Mildew

Yellow upper leaf, grey fuzz below. Improve airflow by spacing heads 6 inches apart and run a fan fifteen minutes every hour. Remove affected leaves immediately; do not compost on-site.

Harvest Windows & Techniques

Cut baby-leaf mixes with sharp scissors 1 inch above soil; expect two more harvests from the same tray. For full heads, slice at the base when firm—not hard—to the touch, usually day 45 for romaine, day 55 for butterhead. Harvest before sunrise; leaves are coolest and sweetest. Dunk immediately in 40 °F well water for five minutes to remove field heat, then spin dry and refrigerate in a perforated bag. Shelf life: fourteen days versus five for store-bought heads.

Succession Schedule for Year-Round Supply

Sow a new tray every ten days. Label each with painter’s tape and a Sharpie—varieties blur when trays line up. Stagger varieties: butterhead in winter (they tolerate lower light), romaine in spring and fall, loose-leaf in summer heat. Maintain a rolling census of 120 heads at different stages inside the house to harvest six heads per week.

Common Mistakes & Quick Fixes

• Leggy seedlings: Lamp too high—lower to 12 inches.
• Bitter taste: Day temps above 75 °F—add shade cloth or harvest earlier.
• Slimy root mat: Over-watering hydroponics—reduce cycles to three per day.
• Pink ribs in romaine: Genetic, not disease; consumer demand now favors it, so no action needed.

DIY PVC Stand for 32 Heads

You can build a two-tier bench from one 10-ft stick of 1-inch schedule-40 PVC, four elbows, four tees, and a hand saw. Cut four 46-inch lengths for the top rails, four 18-inch uprights, and eight 12-inch legs. Dry-fit, mark, then glue with PVC cement. Screw a scrap piece of 1/2-inch plywood on top. Total cost: under thirty dollars, completed in one hour. The open frame stays clean because no soil touches wood.

Companion Planting Inside the Greenhouse

Interplant basil every fourth row; its methyl chavicol odor confuses aphids. Scatter dwarf marigold ‘Tangerine’ along bench edges—roots exude thiophenes that suppress root-knot nematodes. Both companions tolerate identical light and water, so management stays simple.

Water-Saving Hack: Closed-Loop hydroponics

Place the reservoir barrel on the shaded north side. Pipe return lines below bench level so gravity sends used solution back, cutting pump run time by 40 %. Top off with rainwater weekly; replace the entire batch monthly to avoid sodium creep. Typical water use: 1.5 gallons per 50 heads—90 % less than field lettuce.

Bottom Line

A small greenhouse turns lettuce into a perpetual crop. Start with quality pelleted seed, master the cool-temperature regime, and harvest at first firmness. In twelve weeks you will forget the taste of limp supermarket bags and instead crunch into dewy heads you cut five minutes before dinner.

Disclaimer: This article is for general educational purposes only and was generated by an AI language model. Consult local extension services for advice tailored to your climate and regulations.